Greg and Erik, I think that you might have a problem with this. Consider the situation when there is only the rider on board. Suppose he weighs 180 lbs/ 81 kgs. Roughly half that weight will be on the springs with the other half being taken by the front support to the seat. Now put on your favourite pillion passenger say 140 lbs/64 kgs. More or less all their weight will be taken by the rear springs. So the rear loading goes from, say,90 lbs to 230 lbs. That is divided by two as there would be two of the proposed units. If 45 lbs/inch springs were used then they would compress about half an inch each with only the rider on board and about two and a half inches with the pillion passenger on board as well. Even if you pre-load the springs by half an inch so that the solo rider's weight is fully supported prior to a bump, there would still be a further two inches of depression with your main squeeze on board. I think that you would have to build in some adjustment.
Clearly it has been done so it would be good to have feedback from those who have used this as to whether the above problems actually occur.